1. Field of the Invention
The present invention relates to a magnetic disk apparatus having a composite type magnetic head comprising a recording head and a reproducing head, and more particularly to a magnetic disk apparatus in which a magnetic head is positioned in a sector servo system by a reproducing head using a magneto-resistive effect (magneto-resistive effect type reproducing head).
2. Description of Related Art
In general, a composite type magnetic head has high degree of freedom in design because a recording element and a reproducing element thereof are separated from each other. Particularly when a magneto-resistive effect type reproducing head is used to perform a reproducing operation, high reproduction sensitivity can be obtained. Accordingly, a composite type magnetic head having a combination of an inductive head for recording and a magneto-resistive head for reproducing is used in a high-density magnetic disk apparatus.
With respect to the magneto-resistive effect type reproducing head, it is difficult to suppress occurrence of Bark Hausen Noise (hereinafter referred to as "BHN"), and thus it has a disadvantage that distortion of reproduced waveform, variation of reproduced output and/or variation in symmetry of the reproduced waveform occur. In the magneto-resistive effect type reproducing head, the variation of the reproduction output and the variation in symmetry of the reproduced waveform occur due to other causes than BHN. With respect to an abnormal operation phenomenon of a reproducing head, from the viewpoint of "the operational instability of the reproducing head", the abnormal operation due to BHN and the abnormal operation due to the other causes can be identified with each other. Accordingly, in the following description, the abnormal operation which is mainly caused by BHN will be typically described.
Occurrence of BHN varies the value of a position error signal which is required to position a magnetic head, so that the magnetic head is positioned to an incorrect position in a disk radial direction.
The error of the positioning of the magnetic head due to BHN causes the following problems (1) and (2).
(1) If BHN occurs when the magnetic head is positioned during a reproducing operation, a seek error occurs and thus the reproducing operation is impossible. Another problem due to BHN in the reproducing operation than the "position error" problem, is that a phenomenon that a detected error rate is lowered when BHN is superimposed on reproduced data during the reproducing operation of a data area in the magnetic disk. However, these problems are significant only in the reproducing operation, and have no effect on the recording operation itself.
(2) It is required to position the magnetic head to a desired position in the recording operation. Accordingly, even in the recording operation, an operation of decoding a position error signal detected by the reproducing head is necessarily performed. When BHN occurs during the positioning operation of the magnetic head in the recording operation, data is recorded at an erroneous position, resulting in the occurrence of such a critical problem in that the data in the magnetic disk is broken.
In the conventional magnetic disk apparatus, the position error signal required for the positioning is provided with no error correction function. In addition, it is provided with no function of judging whether the position is correct or not. Accordingly, the positioning error as described above can be detected on the basis of only the seek error in the reproducing operation. However, since BHN does not steadily occur, in many cases BHN does not cause a seek error, and only the position error signal indicates an erroneous value.
If BHN causes the seek error, the subsequent recording operation is not performed to avoid the problem in the recording operation (the above (2) problem). However, when no seek error occurs and only the position error signal indicates an erroneous value, data is recorded at an erroneous position, and thus the above problem (2) occurs.
That is, the conventional magnetic disk apparatus (the magnetic disk apparatus having the magneto-resistive effect type reproducing head) has the problem that even when a positioning error occurs due to the variation of the position error signal based on BHN, the erroneous operation (the abnormal operation of the reproducing head) cannot be detected.
In order to avoid the BHN problem in the conventional magnetic disk apparatus, a countermeasure of suppressing the BHN has been considered.
The suppression of the BHN has been performed by a method of controlling the magnetic head. For example, Japanese Laid-open Patent Application No. Hei-5-325110 discloses a method of setting, every magnetic head, a sense current having no BHN produced in the a read amplifier, Japanese Laid-open Patent Application No. Hei-5-258205 discloses a method of applying a bias current to a magneto-resistive head (magneto-resistive effect type reproducing head) when a recording current is applied to an inductive head (inductive type recording head), and Japanese Laid-open Patent Application No. Hei-4-228103 discloses a method of gradually reducing a write-in current in the recording head to reduce BHN in the reproducing operation subsequent to the write-in operation.
These typical techniques have put some technical thought into the suppression of BHN when the magneto-resistive effect type reproducing head is operated.
In addition to the above techniques, there are disclosed many other techniques each of which improves the structure of a magneto-resistive effect type reproducing head or the structure of a composite type magnetic head comprising an inductive type recording head and a magneto-resistive effect type reproducing head to provide a magnetic head having no BHN.
According to the conventional techniques as described above, the level of the suppression of occurrence of BHN can be enhanced. However, it is difficult to perfectly suppress the occurrence of BHN, and when BHN occurs, a malfunction due to BHN (erroneous positioning of magnetic head or the like) cannot be detected. As a conventional countermeasure to the abnormal operation of the magneto-resistive effect type reproducing head due to the other causes than BHN, some improvement has been made on the magnetic head itself, for example by improving the structure of the reproducing head, altering the materials constituting the reproducing head, changing the composition of the materials or the like. Accordingly, no consideration has been taken on "detection of malfunction".
Further, according to an apparatus having an inductive type magnetic reproducing/recording head disclosed in Japanese Post-examined Patent Application No. Hei-8-27888, when a data error is detected during the data reproducing operation from a data area, the recording operation is performed on an area different from the data area concerned, and then the reproducing operation is performed again to enhance effectivity of the reproducing operation. In the composite type magnetic head, the reproducing operation and the recording operation are performed by different elements respectively. On the other hand, in the inductive type magnetic reproducing/recording head, the reproducing operation and the recording operation are performed by the same element. Accordingly, both types of magnetic heads adopt the completely different operating principles for the reproducing operation. Further, there is no disclosure on a technique for moving the magnetic head to another area different from the data area when any error occurs in the data area. This means that the magnetic head itself under trouble must perform the positioning thereof. Therefore, the positioning is not accurately performed, and thus the possibility that the data may be broken is higher. Still further, there is no disclosure of the countermeasure of solving any problem when the problem occurs in the magnetic head during the positioning operation of the magnetic head. Accordingly, in this case, there is a high possibility that the data may be broken.
In the above-described conventional magnetic disk apparatuses, the malfunction due to BHN or the like (erroneous positioning of magnetic head or the like) cannot be detected. Therefore, even when a position error signal indicates an erroneous value due to BHN or the like in the recording operation, the processing is carried out on the assumption that the value is correct, and thus the magnetic head is positioned to an incorrect position. Therefore, the data breakage is surely induced as described in the problem (2).
In the conventional magnetic disk apparatuses, when the reproducing head abnormally works due to BHN or the like in both the reproducing operation and the recording operation, the reproducing head cannot be used continuously. In this case, there is no other way than that of treating the trouble as a failure of the overall magnetic disk apparatus.